In broadband telecommunication networking, there are two emerging standards: synchronous optical network (SONET) and synchronous digital hierarchy (SDH). While SONET is the wideband data networking standard in the United States, SDH is accepted as the international wideband standard for the network node interface. Services based on these fast transmission standards will replace services based on current independently docked plesiochronous networks. SDH defines the international standard for the network node interface, the point where existing plesiochronous signals are adapted to the synchronous hierarchy. SONET is its United States subset. Differences do exist between SONET and SDH; and in addition, both standards are evolving and adding new features. Within SDH, the basic module of data is the synchronous transport module (STM-1) frame. The STM-1 frame is the primary transmission unit of the SDH hierarchy and operates at a speed of 155.520 Mbps. Each STM-1 frame has 9 sets of timeslots with each set having 270 timeslots. Within a time slot interchange unit, a STM-1 frame is converted into a matrix of data bytes with each row being a set of timeslots and a column consisting of all occurrences of an individual timeslot in all rows. The resulting matrix has 9 rows and 270 columns. The STM-1 frame consists of a section overhead, SOH, of approximately 5.184 Mbps and a payload of 150.336 Mbps. The SOH of the STM-1 frame is reserve for SDH management, while the payload contains the information to be transmitted by customers.
STM payloads carry information in the form of virtual containers, VCs, as defined by the CCITT recommendation G.709. Plesiochronous signals from conventional digital networks are adapted to the higher container byte rates by means of inserting (also retorted to as stuffing) bytes. These inserting facilities are provided to adapt the plesiochronous signals to the synchronous network clock. Additional information called path overhead, POH, is added to the container to convert it into a VC. The POH contains information for control, supervision, and maintenance of the transmission network.
To achieve the full flexibility of SDH, a switching system must be able to insert bytes into the STM-1 frames. Given the flexibility that is provided by SDH, it is not always possible to predict which bytes will need to be replaced for future uses of STM-1 frames. Further, the STM-1 frame provides spare channels whose use may well require the insertion of bytes in future applications. In addition, a POH of a VC requires different bytes to be inserted into different STM-1 frames of an STM-1 multiframe. Prior an systems have allowed for minimal flexibility in the bytes to be inserted and have required that the inserted bytes be the same for each frame. Clearly, there exists a need for a system which has sufficient flexibility in the insertion of bytes for protocol purposes that will allow for future applications of SDH.